1. Field of the Invention
The present invention relates generally to magnetic recording systems, and more particularly, to an improved playback circuit which is not subject to a differentiated droop problem.
2. Discussion of the Prior Art
In magnetic recording systems the reading operation in binary recording is concerned with the reconstruction of the original binary input data from the output voltage waveform. Such systems require that each written bit must be properly detected and that false bits must not be interpreted as written bits. Functionally, several detection techniques are available for use in magnetic recording systems.
In one detection scheme the output amplitude of a signal received by a magnetic transducer is sensed. When the level of such signal exceeds a fixed threshold voltage, the presence of a pulse is recognized and the clock timing associated with the signal is resynchronized in a prescribed manner from that time instant. However, in this scheme due to pulse crowding, etc., the signal waveform changes. This causes the instantaneous peak value of the received voltage to occur at varying positions relative to the leading edge of the output pulse. Consequently, the time interval between bits is also caused to vary which is disadvantageous in that the generated clock varies considerably from an optimum timing reference.
In another type of detection scheme, the readback signal is differentiated so that when a peak occurs and the slope changes polarity, the differentiated signal passes through a "zero" reference level. In general, the differentiation approach provides a fairly accurate determination of pulse peak locations. Hence, for a nonperturbed readback signal, pulse timing rather than pulse amplitude determines bit density performance.
An important factor relative to the detection scheme implemented is the code which is used to record or write the data on the tracks of the magnetic media, such as disks, tapes or the like. Presently several codes are available. The most commonly used codes are the frequency modulation (FM) code and the modified frequency modulation (MFM) code. Another code which has been developed but has found only limited use in magnetic recording systems for reasons which will be subsequently described is the modified MFM (M.sup.2 FM) code.
In the FM code, commonly referred to as the "double frequency" code, data bits are written at the leading edge of a bit cell if the bit cell contains a binary unit. Clock bits are written at the leading edge of each bit cell for both binary "one" or "zero". It should be noted that when the FM code is utilized, the detected signal comprises the frequencies f and 2f.
MFM encoding was developed to reduce the high upper frequency required for providing a given amount of data. Data bits are written in the center of a bit cell if the bit cell contains a binary "one". Clock bits are written at the leading edge of a bit cell if the previous bit cell and the present bit cell contain binary "zeros". The frequencies associated with the MFM code are f, f/3f and 2f.
At high packing densities where the data bits are spaced closer together, "bit shift" adversely affects MFM. As magnetically recorded transitions are brought closer together, a magnetic read head will detect both the transition over which it is passing, and the immediately preceding and following transitions, if they are close to the transition being read. Since the transitions alternate in direction, detection of a preceding or following transition subtracts in amplitude from the transition being read. In addition, if only one of the adjacent transitions is close to the transition being read, the subtraction is not symmetrical. Since the amount of subtraction is inversely dependent upon the distance between the transitions, the detection signal for the transition being read will be reduced only on one side. The peak of the detection signal is thus effectively shifted away from the closest adjacent transition. This phenomenon is called "bit shift".
Bit shift may have a disastrous effect upon the separation of MFM information by self-clocking detection circuitry. For example, if a plurality of "ones" are followed by three or more "zeroes", the first clock transition occurs one and one-half bit cells after the last "one" transition and the next clock transition occurs only one bit cell later. The next clock will therefore affect a bit shift on the first clock transition away from the next clock. Hence, the first clock transition encountered by the data separator after a series of "ones" is caused to be erroneously positioned early. The incorrectly positioned clock bit may therefore be erroneously detected as a data bit. Hence, the self clocking circuitry will assume that the detected bit is a late data bit rather than an early clock bit. The timing adjustment is thereby altered erroneously and the system is thrown out of proper timing relationship.
In an attempt to avoid the difficulties encountered in a self clocking code due to bit shift and yet allow recording at relatively high data densities the M.sup.2 FM code was developed. In the M.sup.2 FM code, data bits are written at the center of the bit cell if the bit cell contains a binary "one". Clock bits are written at the leading edge of the bit cell if the previous bit cell did not contain a written transition and the present bit cell contains a binary "zero".
A more complete description of the method and apparatus for communication and storage of binary information in accordance with the M.sup.2 FM code is contained in U.S. Pat. No. 3,560,947, to Robert C. Franchini. It should be recognized that the M.sup.2 FM code includes the frequencies 4/5f, f, 4/3f and 2f, one of which (i.e., 4/5f) is lower than the lowest frequency occuring in the FM or the MFM codes.
However, one of the problems associated with M.sup.2 FM code is that the 4/5f signal tends to cause false detection of bits. For example, at 4/5f the bits are farther apart than they are at f. Consequently, the analog readback signal developed by the magnetic transducer in the magnetic recording apparatus tends to include a shoulder. Typically when such a shoulder occurs, it produces a "droop" in the signal level obtained after the readback signal is differentiated. Since the detector in the playback circuit senses zero crossings of the readback signal, should such a droop approach zero or the detection threshold level, a false bit may be produced.
This problem associated with the development of a false bit occurs primarily in disk drives since the bit density of the information stored on the inside track of a magnetic recording disk is greater than that stored on the outside track due to the difference in circumference between the inside and outside tracks. As a result of the greater spacing between the bits stored on the outside track, the readback signal tends to include shoulders and hence is subject to the previously mentioned "differentiated droop problem" upon the differentiation of such signal.